[Xotcl] xotcl::upvar-compat isn't compatible

Gustaf Neumann neumann at wu-wien.ac.at
Fri May 9 12:01:33 CEST 2008

Jeff Hobbs schrieb:
> Someone asked on chat why the unknown stuff (e.g., typing an unknown 
> command name into the console) in tkcon "broke" after doing:
> package require xotcl::upvar-compat
Thanks for reporting. I rewrote the functions to use catch, as you 
and tried to make clearer what happend in the code. This changes will be in
the next release of XOTcl, and are already checked in the xotcl code 

For the curious, some background about upvar-compat:

Suppose, one has a script like

   Class C

   C method foo {varname} {
     upvar $varname v

   C method bar {} {
     my foo x
     puts "bar x=$x"

   C create c1
   c1 bar 

where a variable name is passed to the method foo. The variable
is in the proc scope of the method var. This example works perfectly fine
with plain tcl upvar, the package upvar-compat is not needed.

However, suppose, one adds an interceptor (e.g. filters, or a mixin class).
For dynamic composition, one should be able to add functionality
(e.g. tracing via filters), without breaking the original code, otherwise
these interceptors are quite useless. Therefore, interceptor transparency
is desired.

Let us take a simple example, adding a per object-mixin

   Class M
   M method foo args {
     set x 200
     puts M.foo.x=$x

   c1 mixin M

In general the intensions is that the upvar linkage from C.foo to C.bar
should not be affected by the mixin, which should be transparent.
However, when c1 bar is called, M.foo lies in the callstack
between C.bar and C.foo. Plain Tcl upvar has no clue, that M.foo
is an interceptor, and refers to M.foo instead of to C.bar. The old behavior
is broken. The situation is even more drastic when filters are used.

The recommended way to achive interceptor transparency is to
use the XOTcl methods upvar and uplevel instead of the Tcl commands.
In the example above, this would be

   C method foo {varname} {
     my upvar $varname v

The package xotcl::upvar-compat changes the global commands
::upvar and ::ulevel to behave like the XOTcl methods, when used
within XOTcl methods.

The approach with the package xotcl::upvar-compat is not recommended,
but supplied for making the application writers life more convenient.

My feeling is that we should drop this package from the xotcl distribution
in the future.

Hope this explains the background.
-gustaf neumann

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